Add missing @FunctionalInterface in oauth2 modules

Fixes gh-8020

oauth2/oauth2-core/src/main/java/org/springframework/security/oauth2/core/OAuth2TokenValidator.java

@@ -1,5 +1,5 @@
 /*
- * Copyright 2002-2018 the original author or authors.
+ * Copyright 2002-2020 the original author or authors.
  *
  * Licensed under the Apache License, Version 2.0 (the "License");
  * you may not use this file except in compliance with the License.
@@ -23,6 +23,7 @@ package org.springframework.security.oauth2.core;
  * @author Josh Cummings
  * @since 5.1
  */
+@FunctionalInterface
 public interface OAuth2TokenValidator<T extends AbstractOAuth2Token> {
 
 	/**